We have documented the presence of filamentous (F-) actin in cystic fibrosis (CF) sputum and observed that actin depolymerizing proteins, especially gelsolin which severs actin filaments, reduce CF sputum's viscosity and elastic shear modulus in vitro. Since some of these actin depolymerizing proteins and their fragments may be suitable for therapeutic use, a better understanding of the role of F-actin and other polymers in the mechanics of purulent sputum as well as of the effects of actin-binding proteins on these mechanics has potential therapeutic value. We will test the general hypotheses that: 1. Agents with different actin depolymerizing mechanisms have different effects on purulent sputum viscosity and therefore work synergistically with each other and with other candidate mucolytic agents to alter rheology of purulent sputum. 2. The viscosity of purulent sputum, its response to actin depolymerizing agents, and its F-actin content are related to the clinical status of patients. Depletion of extracellular actin-depolymerizing proteins may occur in chronic inflammatory airway disease. 3. The polymerization kinetics of actin and actin filament configurations and motions in purulent sputum samples, at rest, during shear stress and following addition of gelsolin and other mucolytic agents modify the rheologic properties of purulent sputum. The first two specific aims test hypotheses critical to understanding the role of actin in purulent sputum rheology and provide information related to the optimization of some potential therapeutic approaches. The last specific aim addresses the fundamental question of how actin alters the viscosity of purulent sputum. A spectrum of rheological and imaging techniques as well as biochemical analyses of sputum will be used to achieve these aims.